Most Helpful Guy
Like Sunmonster says, you get pretty good at judging stuff like that if you do it a lot. I used to work in ski rental, and for that you need to ask people's weight so that you can safely adjust their bindings. After a while, I could make very accurate estimates from just a quick glance. Same thing with being able to look at the shape of a customer's foot, and immediately know which model of ski boot would fit them best.
I'm sure it's the same with sales clerks in clothes stores - an experienced clerk would be able to tell your size at a glance.